Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine's camshaft. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type cam phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type cam phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, as required to meet current or anticipated engine operating conditions.
Knowing the rotational position of the camshaft can be useful, for example, for combustion control and diagnostic functions. In vane-type cam phasers, camshaft position sensing is typically accomplished by using a target wheel rotating with the camshaft which induces a signal on one or more sensors positioned next to the target wheel. The target wheel is disk shaped, and the edge thereof is varied along its periphery in some fashion, for example, by cutting a series of slots along the periphery of the wheel in a predetermined pattern. At least one sensor is used to detect the slots as they pass by the sensor. This type of camshaft rotational position sensing may require one complete revolution in order to synchronize. In other words, it may require one complete revolution in order to sense the pattern of slots to establish the position of the camshaft. Knowing the rotational position of the camshaft more quickly when the internal combustion engine is started or stopped may be desirable.
While vane-type cam phasers are effective and relatively inexpensive, they do suffer from drawbacks. First, at low engine speeds, oil pressure tends to be low, and sometimes unacceptable. Therefore, the response of a vane-type cam phaser may be slow at low engine speeds. Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump, therefore making it more difficult to quickly supply engine oil to the vane-type cam phaser. Third, using engine oil to drive the vane-type cam phaser is parasitic on the engine oil system and can lead to requirement of a larger oil pump. Fourth, for fast actuation, a larger engine oil pump may be necessary, resulting in additional fuel consumption by the engine. Lastly, the total amount of phase authority provided by vane-type cam phasers is limited by the amount of space between adjacent vanes and lobes. A greater amount of phase authority may be desired than is capable of being provided between adjacent vanes and lobes. For at least these reasons, the automotive industry is developing electrically driven cam phasers.
One type of electrically driven cam phaser being developed is shown in U.S. patent application Ser. No. 12/536,575; U.S. patent application Ser. No. 12/844,918; U.S. patent application Ser. No. 12/825,806; U.S. patent application Ser. No. 12/848,599; U.S. patent application Ser. No. 12/965,057; U.S. patent application Ser. No. 13/102,138; U.S. patent application Ser. No. 13/112,199; U.S. patent application Ser. No. 13/155,685; and U.S. patent application Ser. No. 13/184,975; which are commonly owned by Applicant and incorporated herein by reference in their entirety. The electrically driven cam phaser is an electric variable cam phaser (eVCP) which comprises a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine camshaft or an engine crankshaft driven rotationally and fixed to a housing, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an electric motor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. The electric motor may be a brushless DC motor. Brushless DC motors have three or more separate coils and replace the commutator and brushes, which are present in conventional electric motors, with an electronic circuit. Typically, three Hall Effect sensors are used to detect the position of a rotor of the motor. The circuit alternately switches the power on and off in the coils based on input from the Hall Effect sensor inputs, in turn creating forces in each coil that make the motor spin. The Hall Effect sensors are capable of detecting rotor position reliably even at zero RPM as long as the engine controller is still powered.
What is needed is a way to determine the rotational position of a camshaft in an internal combustion engine equipped with an eVCP without the need for additional components. What is also needed is a way to determine the rotational position of a camshaft in an internal combustion equipped with an eVCP even at zero RPM.